The present invention relates generally to drive systems for positive displacement pumps and in particular to a hydraulic drive mechanism for piston pumps.
Reciprocating positive displacement piston pumps have been the standard means of pumping drilling fluids and slurries in the oilfield, the water well drilling industry, the food industry, and in many other applications where high pressures are required and/or highly viscous fluids or heavy slurries are pumped, or where constant or controlled volumes are needed. These mechanically driven reciprocating pumps are dependable but possess a number of limitations and undesirable characteristics. The hydraulically driven piston pumps currently available also possess their own unique limitations and undesirable characteristics.
Mechanically driven reciprocating positive displacement piston pumps that are double acting (that is, they pump in both directions as the piston moves in and out of the pump cylinder) have a rod connected to the piston that forces it fore and aft in the cylinder. The fact that there is a rod on one side of the piston and no rod on the other side creates an uneven displacement as the pumping cycle changes due to movement of the piston from one direction to the other. This causes a surging condition because the flow rate varies up and down with the reciprocation of each piston and causes undesired pressure spikes and inefficiencies in the pumping process.
Further, the mechanical systems used to drive these pumps cause an even worse surging characteristic as a result of the way that the rotating eccentrics drive the pistons back and forth. As these eccentrics rotate, the piston starts with no motion at the end of one stroke through a relatively slow acceleration in travel speed to a relatively high travel speed at the center of the stoke. The piston then decelerates slowing to a movement of no motion at the end of the stroke before it begins motion in the other direction. As a result of these two contributing factors, there are strong surges and pressure spikes in the flow rates of mechanically driven reciprocating positive displacement double acting piston pumps.
The commercially available mechanical pumps described above typically have varying pressure ratings and/or flow ratings depending on the cylinder and piston diameter that is installed in the pump. For example, a given pump equipped with a 6-inch diameter piston/cylinder combination will pump at 300 gpm giving a 250-psi discharge pressure. To reach 1000 psi a 3-inch diameter piston/cylinder combination must be used that results in a flow rate of only 75 gpm. This is because the mechanical drive units are not strong enough to drive 6-inch piston/cylinder combinations at 1000 psi even though the pump chamber is capable of both the high flow rate (300 GPM) and the high pressure (100 PSI).
Mechanically driven piston pumps are very heavy because of the large cast iron housings and gears necessary for them to operate. This excessive weight limits the use of these mechanically driven pumps to unsatisfactory capacities. The designers of such equipment must either compromise other parts of the machine in order to place enough pumping capacity within the machine or compromise pumping capacity to allow for the weight of the components within the machine.
Currently available hydraulically driven mud pumps typically have one hydraulic drive cylinder in the center of the unit connected by its rods to a pump chamber on each end. This stops the surging problems of mechanical pumps, but makes for a relatively long assembly because of the fact that there are pump chambers on both ends as opposed to only a single pump cylinder on one end of the mechanical drive unit.
Hydraulic cylinder driven pumps that have pump chambers only on one end of the drive cylinder suffer the same surge problems of mechanical pumps or worse because of the unmatched displacement ratios between the pump chambers and the hydraulic cylinders. This is due to the fact that the displacement ratios caused by the rod(s) in the hydraulic cylinders and the rod in the one side of the pump chambers are not engineered to compensate for each other.
The inventor knows of no art that attempts to resolve the xe2x80x9cuneven displacementxe2x80x9d problem. There is considerable art in the field of pumps that utilize hydraulic drives for pumps. Evenson (U.S. Pat. No. 4,946,352) discloses a Dual Action Piston Pump. The Evenson system places a hydraulic cylinder at either end of a duplex (two volumetric outputs per complete pump stroke) pump cylinder. Evenson is mostly concerned with a xe2x80x9cvalvingxe2x80x9d arrangement that synchronizes the strokes of the hydraulic cylinders with the pump piston and directs the hydraulic fluid and pump inlet and outlet flows.
Hartley et al. (U.S. Pat. No. 5,505,593) disclose a Reciprocable (sic) Device with Switching Mechanism. Again Hartley is concerned with the synchronizing valve for moving the pump with hydraulic (or other) power. Close examination of the drawings shows that the one of the pump and/or power regions suffers from xe2x80x9cuneven displacement.xe2x80x9d
Peck et al. (U.S. Pat. No. 5,564,912) disclose a Water Driven Pump. Peck is concerned with using a primary fluid pressure source to deliver secondary fluid under pressure. Again close examination of the drawings shows that that the one of the pumps and/or power regions suffers from xe2x80x9cuneven displacement.xe2x80x9d
Thus, there remains a need for an improved hydraulic pump drive system that can provide maximum horsepower over the entire range of the pump; that will provide constant displacement volume with each stroke of the pump, regardless of direction of stroke; that will result in a smaller power unit when compared to standard mechanical units; and that will result in a shorter (and smaller) unit when compared to present state of the art hydraulic drive units.
The instant invention consists of a single hydraulic piston connected to a double acting pump chamber. The hydraulic power cylinder has rods extending from each end of the cylinder both of which are connected to the hydraulic piston within the cylinder. One end is connected to the pump piston (via a connecting rod on the pump) and the other end extends in the opposite direction.
When hydraulic pressure is exerted against the hydraulic piston on the side nearest the pump, the pump piston moves towards the hydraulic cylinder. This action displaces whatever fluid the pump is pumping out of the pump into the pump manifold through a check valve in the normal manner. When hydraulic pressure is exerted against the hydraulic piston on the side furthest from the pump, the pump piston moves away from the hydraulic cylinder. This action displaces whatever fluid the pump is pumping out of the pump into the pump manifold through a check valve in the normal manner.
As stated earlier, the pump stroke away from the hydraulic cylinder will displace more fluid because there is no pump rod within the cylinder. On the other hand, the pump stroke towards the hydraulic cylinder will displace less fluid because the pump rod occupies a given volume within the pump cylinder.
To counter this effect, the diameters of the two rods extending from the hydraulic cylinder are sized so that the ratios of the displacements are matched. This causes even flow of fluid from the pump on each stroke (in or out).
The ends of the hydraulic cylinders that are NOT connected to the pump are used to control hydraulic fluid movement or direction and therefore control the reciprocating movement of the hydraulic drive. The hydraulic valve controls where the hydraulic fluid pressure is appliedxe2x80x94that is, to one or the other side of the hydraulic piston.
The instant invention uses three different methods to control the hydraulic fluid movement or direction. The preferred method uses a hydraulic pilot valve that controls a hydraulic slave valve. The pilot valve is controlled by a simple mechanical link between the hydraulic power cylinder and the pilot valve. The movement of the hydraulic cylinder switches the direction of hydraulic fluid through the pilot valve that in turn switches the direction of applied hydraulic fluid to the hydraulic cylinder resulting in reciprocation of the piston within the hydraulic cylinder as long as hydraulic energy (power) is applied.
An alternate embodiment uses a mechanical switching means that is connected to the hydraulic rod opposite to the pump. As the hydraulic piston moves away from the pump and reaches the maximum point, it trips a mechanical limit that positions the control valve to apply hydraulic fluid to the other side of the cylinder. In a similar manner, as the hydraulic piston moves away from the pump and reaches the maximum point, it trips another mechanical limit that positions the control valve to apply hydraulic fluid to the first side of the cylinder. Thus, the hydraulic drive system will reciprocate as long as hydraulic power is supplied to the unit.
A further alternate embodiment uses a proximity switch, instead of mechanical limits, to control a solid-state relay that controls the application of hydraulic power to the cylinder via an electric solenoid valve. Therefore, in a like manner, the hydraulic drive system will reciprocate as long as hydraulic power is supplied to the unit.